The specific aims of the research proposed here are to describe in detail the chemical mechanism for covalent bond formation between both the third (C3) and the fourth (C4) human complement proteins and the variety of molecules with which they can form either acyl ester or amide bonds and to determine the chemical nature of interactions between these proteins and antigen and antibody molecules. The activation of C3 and C4 and their subsequent covalent binding to immune complexes are key steps in the processing and clearance of the complexes. Hence, the chemical reactivity of these complement proteins with antigen and antibody molecules is of special concern, particularly in view of the finding that genetically different forms of C3 and C4 are known to be associated with certain auto-immune diseases of which systemic lupus erythematosus is an example. Indeed, in view of our current knowledge of the chemical and molecular properties of C3 and C4 it is appropriate at this time to focus our attention on their interactions with antigen and antibody molecules in relation to immune complex-mediated disease in general. Accordingly, we will investigate 1) the mechanism of covalent bond formation and hydrolysis, 2) the chemical nature of covalent bonds formed between C3 and C4 and model immune complexes composed of ovalbumin and anti-ovalbumin, 3) the covalent interactions between C3 and C4 and DNA antigens, 4) the covalent interactions between C3 and C4 and the capsular polysaccharide antigen of Haemophilus influenzae, type b, and 5) the use of anti-C4 monoclonal antibodies as probes of C4 structure and function.